LNAs are a well-known type of electronic amplifier used in communication systems to amplify very weak signals captured by an antenna. They are typically located at the antenna and are designed so as to add very little noise to the received signal. The LNAs amplify the received signals to a level required by subsequent receiving equipment to which the LNA is attached. They may also be referred to as signal boosters.
One known application of LNAs is to receive and amplify direct broadcast satellite (DBS) signals, and LNAs adapted for this purpose may be referred to as DBS LNAs. DBS LNAs typically comprise a number of FETs (which may be GaAs FETs) for processing radio frequency (RF) signals. For example, the DBS LNA may be adapted to receive signals having two different polarisations, and two FETs may be used to select which one of the input signal polarisations is amplified and passed on to subsequent connected equipment. Also, a FET may be arranged in an active mixer circuit to receive an RF input, with the gate or drain of the FET being driven by a signal from a local oscillator in the LNA. The active mixer circuit is then able to output (i.e. extract) an intermediate frequency (IF) signal.
DBS LNAs are typically required to detect very low-level RF signals covering a wide frequency range, and provide high channel-to-channel isolation. They should also be able to amplify received signals whilst introducing negligible noise, and be controllable to select between different input signal polarisations (as discussed above). It is known for them to be controllable to band switch so as to be able to receive and process signals over an increased frequency range. It is known for them to be able to down convert, that is to be able to receive an input signal at a particular frequency and output a corresponding signal at a lower frequency. Another feature of known DBS LNAs is cable drive, that is the LNA is powered and controlled via the same RF cable that is used for downfeeding the RF output of the LNA to connected equipment (such as a “set top box”).
In the past, DBS LNAs have typically incorporated a number of separate internal circuit blocks on a printed circuit board (PCB), these blocks including: a block providing FET bias control and protection stages; a block arranged to generate a negative supply voltage for use in FET control; a block arranged to detect the level of a DC input voltage for use in polarisation switch control; a block arranged to detect an AC input voltage for use in band switch control; a block arranged to control switching of power to local oscillators, and block arranged to provide a regulated power supply. These numerous separate blocks in known LNAs had to be accommodated on relatively large area PCBs, and have taken up 50% or more of the total LNA PCB area. This has added cost to the overall LNA, this cost not just being associated with the separate components and the PCB (which has typically been made from expensive low-loss RF material), but also the LNA housing material (alloy and plastic).